This link explains offset with a drawing. It's the distance the inside wheel mounting surface is from the center line of the wheel. http://www.tirerack.com/wheels/tech/...jsp?techid=101
Note the link in my first response has the offset listed in the right column. They are listed as H high M medium L low. You should try to stay close to the original offset for best handling and less stress on the chassis components when changing wheels.